So far watchdog users could only configure the watchdog with the highest
priority. In preparation for having the wd command configure a watchdog
by name, extend watchdog_set_timeout with a struct watchdog *parameter
and export functions to query default watchdog and to find watchdog by
name.
No functional change.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>